1. Field of the Invention
This invention relates to the field of materials testing, and in particular to a laser-ultrasonic system for the ultrasonic testing of objects or characterizing of materials with ultrasound.
2. Description of Related Art
Laser-ultrasonics is an emerging technique for the nondestructive evaluation of objects and materials, which has several advantages over other nondestructive techniques. A typical laser-ultrasonic inspection system is composed of two lasers and a phase or frequency demodulator, as shown in FIG. 1. In laser-ultrasonics, the generation of ultrasound is performed at a distance, which in practice can range from a fraction of a meter to several meters. The source of ultrasound is the surface of the material itself and detection of ultrasonic motion is performed off the same surface, which eliminates the coupling liquid and perpendicularity requirements of conventional ultrasonics.
Laser-ultrasonics can be used on parts of complex shape and at elevated temperatures. The laser-ultrasonic technology has been demonstrated to be applicable to real industrial conditions. In particular, a system has been developed for measuring on-line the wall thickness of steel tubes at 1000xc2x0 C. moving at 4 m/s. Systems have also been developed for the inspection of aircraft parts with very complex geometries and made of composite materials. Many other applications are presently explored and have been presented in various conferences on ultrasonics, optics or non-destructive testing.
In order to be useful in practice laser-ultrasonics generally requires strong ultrasound generation and sensitive detection. Various techniques are known to improve generation strength and have been described in the literature. One technique consists in using material ablation. This has the disadvantage of causing some mate rial damage. Another technique involves using a laser with a wavelength that provides light penetration below the material surface.
Many interferometric detection schemes are known. Optical detection of ultrasound is based on the demodulation of the small phase or frequency shift imparted on the light from the detection laser scattered by the surface in ultrasonic motion. The detection schemes can be sensitive to the speckle of the scattered light (such as in U.S. Pat. No. 4,633,715 by J.-P. Monchalin entitled xe2x80x9cLaser Heterodyne Interferometric Method and System for Measuring Ultrasonic Displacementsxe2x80x9d) or insensitive to the speckle such as the scheme based on a confocal Fabry-Perot interferometer (U.S. Pat. No. 4,659,224 by J.-P. Monchalin, entitled Optical Interferometric Reception of Ultrasonic Energy, U.S. Pat. No. 4,966,459 by J.-P. Monchalin entitled Broadband optical detection of transient surface motion from a scattering surface, U.S. Pat. No. 5,137,361 by R. Hxc3xa9on and J.-P. Monchalin entitled Optical detection of a surface motion of an object using a stabilized interferometric cavity and U.S. Pat. No. 5,080,491 by J.-P. Monchalin and R. Hxc3xa9on entitled Laser optical ultrasound detection using two interferometer systems). Insensitivity to the speckles means that demodulation is insensitive to the wavefront of the scattered wave. In other words, demodulation occurs effectively on a large number of speckles (in contrast with the speckle sensitive schemes that work best with one speckle) and that the demodulator has a large etendue or throughput. These terms mean that the demodulator can effectively demodulate light coming from a large illuminated detection spot and received through a large aperture.
Insensitivity to speckle can also be realized by adaptation of the reference wave of the interferometer in a non-linear optical element, which is usually in practice a photorefractive crystal. A holographic grating is written inside the crystal by interference of the wave scattered by the surface and a pump wave directly derived from the detection laser. This grating then diffracts a reference wave with a wavefront adapted to the one of the received scattered wave. Such a scheme (two-wave mixing) is described in U.S. Pat. No. 5,131,748 by J.-P. Monchalin and R. K. Ing entitled Broadband Optical Detection of Transient Motion from a Scattering Surface and U.S. Pat. No. 5,680,212 by A. Blouin, P. Delaye, D. Drolet, J.-P. Monchalin, G. Roosen entitled Sensitive and fast response optical detection of transient motion from a scattering surface by two-wave mixing. This scheme provides also automatically frequency tracking to drifts or changes of frequency of the detection laser (within the response time of the two-wave mixing interferometer). It does not require a stabilization electrical network to lock the laser frequency to the interferometer as the confocal Fabry-Perot based detection schemes.
Other adaptive two-beam mixing demodulators also present similar properties of speckle insensitivity like the photo-emf based demodulator proposed by M. P. Petrov, I. A. Sokolov, S. I. Stepanov, G. S. Trofimov, Non-steady-state photo-electromotive-force induced by dynamic gratings in partially compensated photoconductors in J. Appl. Phys. 68, 2216, (1990) or more recently the demodulator based on the polarization self-modulation effect by K. Pxc3xa4ivxc3xa4saari, A. A. Kamshilin, Adaptive sensors of rough-surface ultrasonic vibrations based on the polarization self-modulation effect, Fourth International Conference on Vibration Measurements by Laser Techniques: Advances and Applications, SPIE Proceedings vol. 4072, 70, (2000).
In spite of these advances in speckle insensitive demodulation that make optical detection of ultrasound more practical for detection off industrial surfaces that are usually rough, they do not ensure that the technique is sufficiently sensitive, particularly when the ultrasonic signals are very weak (e.g. thick specimens and ultrasonically absorbing objects), when the surface is strongly absorbing light (e.g. all black carbon-epoxy composite materials) and when detection has to be performed meters away (e.g. objects at elevated temperature and inspection over large aircraft parts). In all these cases, in order to have adequate sensitivity the detection laser has to be powerful. Kilowatts peak power often gives only milliwatts at the interferometer level because of the many losses encountered when going from the laser to the demodulator. Such a power would be in practice hardly feasible if needed continuously; fortunately, it is only required from time to time at the repetition rate of ultrasound generation and over a time window in the range of 1 xcexcs to a few 100 xcexcs, depending upon the propagating time of ultrasound in the object or at its surface. However there are in addition severe stability criteria, in frequency or phase and intensity, since the laser should not introduce on the detector noise above the shot noise.
Current practice is to start from a very stable cw (continuous wave) low power (typically 100 mW) Nd-YAG laser oscillator and to amplify it to the desired peak power with several Nd-YAG pulsed amplifiers or using several passes in one amplifier or a combination of both. The amplifiers can be flashlamp pumped or laser diodes pumped. Nd-YAG and a few other materials are capable of providing the high gain needed. The cw low power laser oscillator is typically monolithic and pumped by a laser diode and has by design the stability requirements. A stabilization loop is often used to minimize the relaxation oscillations that appear in intensity and in phase, improving further stability.
Commercial products are in particular available from Lightwave Electronics in California and InnoLight in Germany. Amplification maintains the stability properties of the low power cw laser oscillator, resulting in a high peak power output with the desired phase or frequency and intensity stability. A typical multi-amplification stage system is shown in FIG. 2. Each stage usually includes a laser rod (flashlamp pumped or laser diode pumped) and is doubled pass. Permanent magnet Faraday isolators are added between the cw laser oscillator, between stages and at the output to prevent parasitic oscillation (i.e. lasing) of the whole system. FIG. 3 shows a typical zig-zag multipass slab system. Such a system is usually laser diode pumped. Only two passes are shown for sake of clarity.
U.S. Pat. No. 5,608,166 describes the use of a long pulse detection laser, but this is not suitable for objects noted above that provide very weak ultrasonic signals and require a high power laser. Such high power lasers are too noisy to provide adequate sensitivity for very weak ultrasonic signals.
It is readily apparent that these existing detection laser systems are very complex, have a very large footprint and in turn have a high cost, which limits widespread use of the laser-ultrasonic technique. There is a need for a simpler, more compact and less costly detection system to be used concurrently with a suitable demodulator (preferably speckle insensitive).
According to the present invention there is provided a method for ultrasonic testing of objects comprising the steps of generating ultrasound inside or at the surface of the object; illuminating the surface of the object with an incident beam from a long-pulse laser oscillator that is modified to be substantially free of intensity fluctuations; collecting light from said beam that is scattered or reflected by the surface of the object; and demodulating the scattered light with a frequency tracking demodulator to obtain a signal representative of the ultrasonic motion.
A long pulse laser typically has a pulse duration in the range 1 xcexcs to a few 100 xcexcs. Long pulse lasers that provide the desired peak power with a pulse duration in the 100 xcexcs range are known to be very noisy and are typically affected by strong relaxation oscillations and even spiking. The applicants have found surprisingly that if steps are taken to reduce intensity fluctuations if the proper demodulation is used, an effective solution to the problem is provided.
In a preferred embodiment, the long pulse laser oscillator is concurrently used with a speckle insensitive phase demodulator for the optical detection of ultrasound.
In another aspect the invention provides an apparatus for the ultrasonic testing of an object comprising an ultrasound generator for generating ultrasound inside or at the surface of the object; a Long pulse laser oscillator that is substantially free of intensity fluctuations for generating an incident beam for illuminating the surface of the object; a tight collector for collecting Light from said beam that is scattered or reflected by the surface of the object; and a frequency tracking demodulator for demodulating the collected tight to obtain a signal representative of the ultrasonic motion.